1. Field of the Invention
The present invention relates to the structure of an n- or p-channel type thin-film transistor, a method for manufacturing an n- or p-channel type thin-film transistor, and a display using such thin-film transistors.
2. Description of the Related Art
Amorphous silicon thin films and polysilicon thin films have been used as semiconductor thin films used to form, for example, thin-film transistors (TFTs) serving as switching elements that control voltages applied to pixels in a liquid crystal display (LCD) or thin-film transistors for a control circuit for the liquid crystal display.
In TFTs using polysilicon thin films as semiconductor thin films, electrons or holes migrating through a channel region generally have a higher mobility than in TFTs using amorphous silicon thin films as semiconductor thin films. Accordingly, the transistors using polysilicon thin films have higher switching speeds and can thus operate faster, than the transistors using amorphous silicon thin films.
This enables TFTs to be used to form an LCD pixel selection circuit and a peripheral drive circuit to be formed on the same substrate on which pixel control thin-film transistors are formed, the peripheral drive circuit driving an LCD. Further, the design margin of other parts can be advantageously increased. A cost and size reduction and an increased definition can also be achieved by incorporating the peripheral drive circuit such as a driver circuit or DAC into a display section including the pixel control thin-film transistors.
The present applicant has developed a mass production technique for stably manufacturing a large-grain-size crystallization region in a non-single-crystal semiconductor thin film formed on an insulating substrate. As a method for forming a large-grain-size crystallization region, crystallization methods have been proposed in, for example, “Method for Forming Giant Crystal Grain Si Film Using Excimer Laser”, Masakiyo MATSUMURA, Surface Science, Vol. 21, No. 5, pp. 278 to 287, 2000, and “Method for Forming Giant Crystal Grain Si Film Using Excimer Laser Light Irradiation”, Masakiyo MATSUMURA, Applied Physics, Vol. 71, No. 5, pp. 543 to 547, 2000. Successful mass production of a large-grain-size crystallization region enables not only the liquid crystal display section and switching transistors for the pixels but also a memory circuit such as DRAM or SRAM, an arithmetic and logic circuit, or the like to be formed on a glass substrate. This enables a reduction in the amount of power required by the entire liquid crystal display and its size.
The present inventor et al. have developed a manufacture technique for forming higher-performance TFTs that offer practical, optimum transistor characteristics. For example, a single-crystal silicon having a crystal with a large grain size grown by executing a thermal treatment on an amorphous silicon thin film has a surface different from that of a single-crystal silicon wafer formed by slicing a single-crystal rod formed by a normal lift-off method. Specifically, the former single-crystal silicon has a thin film that is not microscopically flat and has a complicated grain boundary generated during crystal growth.
It has thus been found that a desired off-current characteristic is not obtained simply by forming a TFT at an arbitrary portion in the crystallization region. It has also been found that a desired mobility transistor is not obtained.
The present invention is intended to solve the problem of the degraded transistor characteristics to provide a TFT structure offering the optimum off-current and mobility characteristics, a method for manufacturing such a TFT, and a display using such a TFT.